1. Field of the Invention
The present invention relates to a rotary connector.
2. Description of the Related Art
Conventionally, a rotary connector is used for electrically connecting power lines, signal lines, and the like, between two relatively rotating members (for example, see Japanese Patent Application Laid-Open (kokai) No. H5-82223). This kind of rotary connector can maintain electrical connection irrespective of a relative rotation angle of the rotating members.
FIG. 13 is a plain view of the main part of a conventional rotary connector.
In FIG. 13, a reference numeral 301 designates an inner ring made of electrically conductive metal, which is connected to a wire extending from one member on which the rotary connector is mounted. Also, a reference numeral 302 designates an outer ring made of electrically conductive metal, which is connected to a wire extending from the other member on which the rotary connector is mounted. In this case, the inner ring 301 and the outer ring 302 are positioned to form concentric circles, and the one member and the other member relatively rotate around the central axis of the inner ring 301 and the outer ring 302.
Also, a circular retainer 303 is rotatably placed relative to the inner ring 301 and the outer ring 302 between the inner ring 301 and the outer ring 302. Wheels 304 made of electrically conductive metal are mounted in the retainer 303. The wheels 304 are rotatably mounted relative to the retainer 303 at three points in the retainer through mounting shafts 305.
The wheels 304 roll along the outer circumference surface of the inner ring 301 and the inner circumference surface of the outer ring 302 when the inner ring 301 and the outer ring 302 relatively rotate. Thereby, the wheels 304 can electrically connect the relatively rotating inner ring 301 and outer ring 302 irrespective of the rotation angle therebetween.
However, since the wheels 304 in the conventional rotary connector are rigid and do not deform in the radial direction thereof, electrical connection can be momentarily broken between the inner ring 301 and the outer ring 302. Theoretically speaking, the inner ring 301 and the outer ring 302 are always electrically connected through the wheels 304, if the wheels, which have a diameter equal to a difference between the radius of the outer circumference of the inner ring 301 and the radius of the inner circumference of the outer ring 302, are rotatably placed in equally-spaced three points between the outer circumference of the inner ring 301 and the inner circumference of the outer ring 302. However, in practice, dimensional errors in manufacturing and assembling the inner ring 301, the outer ring 302, and the wheels 304 causes backlash between the inner ring 301, the outer ring 302, and the wheels 304. This causes all the wheels 304 to be separated, even though momentarily, from the outer circumference surface of the inner ring 301 or the inner circumference surface of the outer ring 302, and thus may result in electrical disconnection in some cases.
Therefore, in order to provide higher reliability in electrical connection, Japanese Patent Application Laid-open (kokai) No. H5-82223 discloses a rotary connector, in which flange portions in the outermost circumferences of each of the wheels 304 slidingly contact the side surfaces of both the inner ring 301 and the outer ring 302 in such a way as to hold the side surfaces thereof between the flange portions. However, since the flange portions of each of the wheels 304 slidingly contact the side surfaces of the inner ring 301 and the outer ring 302 on both sides thereof, substantial resistance occurs to relative rotation of the inner ring 301 and the outer ring 302. Moreover, it causes wear of the flange portions of the wheels 304 or of the side surfaces of the inner ring 301 and the outer ring 302, and thus bad electrical contact occurs after long-term use.